Looks like the issue with the display of these icons is in the (incorrect?) Alpha channel present. Also, mogrifiing * .tga to * .tga with removed alpha does not work out for some reason, so I had to transform it to dds. I am glad, though, that it's not required to patch mod itself to make things work.

So, I made simple plugin to test . I placed an example icon into Data Files/Icons folder, and in vanilla CS changed icons of 2 magic effects which belongs to Imperial race abilities. In vanilla MW this icons were changed and correctly displayed in race selection dialogue, in OpenMW 0.43 on Windows, icons were missed, without any fallback images, just transparent. Can anyone else confirm that, please? icon_test.zip

Not sure if you still need any testers, but this is what I see when I look at Star of the West with your plugin loaded.